1. Field of the Invention
This invention relates to a subscriber station in a subscriber radio system, where a plurality of subscriber stations communicate with a base station through a time division multiplexed radio wave having a specific frequency.
This type of system is utilized, for example, in a radio communication system for specific subscribers using a portion of a public telephone line.
2. Description of the related Art
In the aforementioned system, each subscriber station is required to communicate with the base station within a time slot assigned to each subscriber station. Therefore, a signal having a frequency for determining the time slot has to be generated in the subscriber station.
If all the subscriber stations comprise frequency generators having high stability similarly to the base station, to perform stable radio communication within the assigned time slot, the cost of the subscriber station would be very high, so that the subscriber's expense and the cost of the whole system would be increased.
A system where a low-priced frequency generator is used in the subscriber station and a frequency of the generator is controlled depending on the transmitting frequency of the base station (i.e., down frequency), is employed.
In the conventional subscriber radio system, each subscriber station comprises a closed loop, for example, a PLL loop, which synchronously controls the frequency of the generator depending on the transmitting frequency of the base station. Since a received frequency in the subscriber station varies according to the state of a radio transmission path between the basic station and the subscriber station, the frequency of the generator varies following the variation of the received frequency. When the distance between the base station and the subscriber station is large, the influence on the state of the radio transmission path is so large that the closed loop is liable to become unstable in subscriber stations far from the base station. If the closed loop becomes unstable, the frequency of the generator varies extensively. Thus a transmitting radio wave from the subscriber station invades time slots of other subscriber stations to interfere with transmitting radio waves from the other subscriber stations, since the time slot occupied by a subscriber station is determined based on the frequency of the generator. Namely, an extreme oscillation in a subscriber station incurs disorder in not only the subscriber station itself, but also in the overall system.